Hi davidtee !
I had the same problem when I plugged everything together today with an HDMI cable, using the TV's HDMI-1 socket. My TV is also a Panasonic (but not a 'SmartTV').
After much frustration, and tea, I tried the volume controls on the TV's own remote-control (not the TalkTalk one) and there was the sound! So I turned it up to 90 and shall leave it there, and now I can control the volume with the TalkTalk remote-control. The 90 is just my choice, but I decided on it so I can have a range of volume with the TalkTalk remote.
I hope it works for you.
(Care: if ever you use the TV without the TalkTalk-stuff plugged in it will be very loud.)
A different thing you might find: nothing happens when I use the 'Menu' button on the TV's-remote (to adjust the picture), only the Quick Menu button. I had adjusted the TV before plugging the TT-box in and if I need to change anything I hope I can if I unplug the TT-box first.